1. Field of the Invention
Organomodified siloxanes, in particular polyethersiloxanes are widely used compounds in virtually all areas in which control of surface-active behavior is important. The range of use of this class of substances is based not least on the possibility of establishing a variety of action principles in a controlled manner by a suitable combination of siloxane skeleton and polyethers as substituents.
The standard reaction for the organomodification of hydrogen siloxanes is platinum-metal catalyzed hydrosilylation. The industrial synthesis of the Si—C-linked polyethersiloxanes is based to a considerable extent on the use of the readily available allylpolyether.
In the case of the polyether-modified siloxanes, prepared by the process described above from allylpolythers and hydrogen siloxanes, considerable excess amounts of the polyether components are used in some cases to ensure a quantitative SiH conversion and hence to avoid evolution of H2 from the end product. This procedure is explained by the fact that, in addition to the desired Si—C linkage, varying degrees of isomerization of the allylpolyether used to give the corresponding, thermodynamically more stable propenylpolyether are observed. The propenylpolyether is not accessible to an Si—C linkage under the customary conditions of hydrosilylation.
However, undesired properties of the polyethersiloxanes result from the presence of propenylpolyether. Under the influence of traces of acid and moisture, the propenylpolyether undergoes hydrolysis; in other words, propionaldehyde is liberated over a certain period. As a result of secondary reactions, linear and/or cyclic oligomers (aldoxanes, trioxanes) also readily form from the propionaldehyde and have a tendency to cleavage and hence for the liberation of aldehyde again. Products which are required to have a neutral odor (for example for applications in the cosmetics sector) therefore require an aftertreatment.
If, moreover, compounds carrying hydroxyl groups (for example, siloxane-bonded polyetherols) are contained in the aldehyde-contaminated system, acetals which may distort the physicochemical property profile of the desired product, for example by increasing the viscosity as a result of the increase in molar mass, etc., may readily form.
2. Description of the Related Art
The prior art discloses very different methods for avoiding or eliminating the problems described in the case of allylpolyether-based systems:
EP-A-0 118 824 describes organopolysiloxane/polyoxyalkylene copolymers as oils for cosmetic purposes, having a total content of compounds carrying carbonyl groups (aldehydes+ketones) of ≦100 ppm and a peroxide content of ≦5 milliequivalents/kg of substance, which are obtainable by using antioxidants in amounts of 5 to 1000 ppm, if required in the presence of a buffer during the hydrosilylation linkage reaction of allylpolyethers which are already very pure.
JP-A-07304627 teaches a process for the treatment of organosiloxanes carrying allylpolyethers by mixing them with aqueous hydrochloric acid at 60° C. in the course of 24 hours. The aldehyde content obtained is ≦100 ppm and the odor test is negative.
A comparable procedure for the acid-induced hydrolysis of propenylpolyether moieties with liberation, and the consequently possible removal of propionaldehyde is described in J. Soc. Cosmet. Chem. Japan (1993), 27(3), 297-303.
DE-A-41 16 419 relates to the elimination of undesired odor sources in the allylpolyethersiloxane by heterogeneously catalyzed hydrogenation under pressure over nickel/kieselguhr catalysts, colorless transparent products without a penetrating odor being obtained, which are stable in the aqueous acidic system and in a pH range of from 3 to 4 for a period of 6 weeks.
EP-A-0 398 684 describes the preparation of polyoxyalkylene/silicone block copolymers having little odor by reacting a hydrogen siloxane with allylpolyethers in ethanol under Pt catalysis and treating the reaction mixture with a dilute hydrochloric acid solution at elevated temperatures for a few hours and then subjecting it to a vacuum distillation, a virtually odorless copolymer being obtained.
The prior art furthermore describes the possibility of suppressing undesired odor in allylpolyethersiloxanes by adding small amounts of phytic acid, which however remains in the system (JP-A-60018525).
The indirect routes taken to avoid the problems resulting from the allylpolyether isomerization are disclosed, for example, in EP-A-0 308 260, which claims a process for the preparation of highly pure oxyalkylene-modified organopolysiloxanes using vinyloxy-terminated polyethers. Because of limited availability and high raw material costs, this preparation route cannot be extended as desired.
JP-A-09012723, too, makes use of an avoidance strategy which replaces the hydrogen atoms in position 3 of the polyether-bonded allyl group by hydrocarbon substituents. Of course, a system modified in this way does not suffer from any allyl-propenyl rearrangement during the hydrosilylation.
On evaluating all these processes, it is found that no process is suitable for use for all allylpolyethersiloxanes in a very wide range of applications. Additives inherent in the system, such as antioxidants and complexing acids (phytic acid), prevent the use of the copolymers treated in this manner in various applications, for example in the cosmetics or paint sector.
Processes such as the heterogeneously catalyzed hydrogenation under pressure are complicated and expensive and hence acceptable only for small-volume, high-priced application areas of the silicone polyethers. If in particular those silicone polyethers which are used in the form of foam stabilizers in the preparation of polyurethane foams and have a complex structure are included in this consideration, insufficiencies of an acid treatment or of a combined acid/alcohol treatment of the corresponding block copolymers are also evident.
Attempts to treat these functional surfactants with acid under moderate conditions illustrate the disastrous effect of this method on the suitability for use as foam stabilizers, in particular in hot flexible foam systems. Instead of the desired foam stabilization, a collapse of the labile foam structure is observed.
JP-A-09095536 is concerned with the preparation of highly pure siloxanes containing oxyalkylene groups as modifiers for polyurethanes. The adducts of very short-chain allyl-ethyleneoxy-propyleneoxy-ethers having (M≦250 g/mol) with hydrogen siloxanes are considered here. After the end of the addition reaction, the crude product is subjected to a treatment with stripping gas at 150° C. and reduced pressure (5 mmHg). These conditions are sufficient for virtually completely removing unconverted low molecular weight ethers, such as, for example, propylene glycol monoallyl ether, from the end product. In the case of the allylpolyethersiloxanes whose polyether base comprises molar masses of about 400-10 000 g/mol, however, propenyl-containing moieties cannot be eliminated permanently and completely in this manner.